The present invention relates generally to a super-conducting coil in a synchronous rotating machine. More particularly, the present invention relates to a cryogenic gas coupling between a source of cryogenic fluid and the rotor of the machine.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher, which increase the power density of the electrical machine and result in significant reduction in weight and size. Air-core super-conductive rotors, however require large amounts of super-conducting wire, which adds to the number of coils required, the complexity of the coil supports, and the cost.
High temperature SC coil field windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K, to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
Super-conducting coils have been cooled by liquid helium. After passing through the coil windings of the rotor, the hot helium is returned from the windings as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium. This reliquefaction poses significant reliability problems and requires significant auxiliary power for cryorefrigeration.
Prior cooling techniques include cooling an epoxy-impregnated SC coil through a solid conduction path from a cryocooler. Alternatively, cooling tubes in the rotor may convey a liquid and/or gaseous cryogen to a porous SC coil winding that is immersed in the flow of the liquid and/or gaseous cryogen. Immersion cooling requires the entire field winding and rotor structure to be at cryogenic temperature, as a result no iron can be used in the rotor magnetic circuit because of the brittle nature of iron at cryogenic temperatures.
A cooling fluid coupling is needed to connect the stationary cryorefrigeration unit to the rotor and its SC coils. The coupling must transfer inlet and outlet cooling fluid between a stationary source and the rotating end shaft of a rotor. Contact seals are commonly used in transfer couplings for cryogenic cooling systems connected to rotors and other rotating components. Contact seals have increased frictional losses that degrade cryorefrigerator capacity, and limit the life and reliability of the coupling because of seal wear. Relative motion gap seals have also been used in transferring cooling fluid to a rotor. However, relative motion gap seals have high heat transfer losses. Extended thermal standoff lengths for the relative motion gaps have been used to reduce heat transfer losses to the cryogenic gas and to improve cryorefrigerator capacity. However, these long thermal standoff lengths result in long overhung tubes that may vibrate excessively and come into rubbing contact with the rotor of the generator. Accordingly, there is a long-felt need for better cryogenic gas couplings with a rotor.
Heat transfer losses with respect to the cryogenic gas cooling system for the HTS coils should preferably be minimized to conserve refrigeration power. The coupling between the stationary cryogenic gas source and the rotor of the synchronous machine is a potential source of cryogenic gas leakage. To minimize gas leakage at the coupling, it is desirable that the leakage between the inlet and return gas streams be minimized, and that adequate thermal insulation be provided between the cryogenic gas and surrounding ambient temperature components. In addition, the operational life and high reliability of the transfer coupling should be commensurate to the expected life and reliability of the synchronous electrical machine.
A cooling gas coupling has been developed to connect a supply of cryogenic gas (or cooling fluid) to the shaft of a rotor in a synchronous electrical machine. Cooled cryogenic gas (or other fluid) is transferred from a stationary cryorefrigerator through a stationary bayonet to a tube rotating with the rotor having a HTS coil winding. The cooling gas transfer occurs using a cryogenic gas transfer joint attached to the collector end of the rotor. A relative motion gap created with a clearance seal about a bayonet coupling limits leakage of the inlet cooling gas to the lower pressure return gas, and a relative motion gap over a length of the rotating return tube provides thermal insulation to the return cryogenic gas.
In a first embodiment, the invention is a cooling fluid coupling for providing cooling fluid to a rotor having a super-conducting winding of a synchronous machine and a source of cryogenic cooling fluid. The fluid coupling comprises an inlet cooling tube and an outlet cooling tube in the rotor and coaxial with an axis of the rotor. The inlet cooling tube has an input port coupled to receive inlet-cooling fluid from the source of cryogenic cooling fluid. The outlet cooling tube has an output port coupled to return cooling fluid from the rotor to source. A rotating motion gap seal separates the input port and output port of the coupling.
In another embodiment, the invention is a cooling fluid coupling between a rotor for a synchronous machine and a source of cryogenic cooling fluid. The coupling comprises: (i) a rotating inlet cooling tube and a rotating outlet cooling tube in the rotor and coaxial with an axis of the rotor; (ii) the inlet cooling tube is coupled to receive inlet cooling fluid from the source of cryogenic cooling fluid; (iii) the outlet cooling tube is coupled to return cooling fluid from the rotor to source, and (iv) a rotating motion gap seal supports the inlet cooling tube in the outlet cooling tube.
In a further embodiment, the invention is a cooling fluid coupling between a rotor for a synchronous machine and a source of cryogenic cooling fluid. This coupling comprises: (i) a rotating inlet cooling tube and a rotating outlet cooling tube in the rotor and coaxial with an axis of the rotor; (ii) the inlet cooling tube is coupled to receive inlet cooling fluid from the source of cryogenic cooling fluid; (iii) the outlet cooling tube is coupled to return cooling fluid from the rotor to source; (iv) a rotating non-contact motion gap seal supporting the inlet cooling tube in the outlet cooling tube; (v) a stationary third tube encircling the outlet cooling tube and said third tube supported by a bearing, and (vi) a magnetic field seal supporting the outlet cooling tube in the stationary third tube.